Glutamate is the primary excitatory amino acid in the mammalian central nervous system.
Neurotransmission mediated by glutamate has been demonstrated to be critical in many physiological processes, such as synaptic plasticity, long term potentiation involved in both learning and memory as well as sensory perception (Riedel et al., Behay. Brain Res. 2003, 140:1-47). Furthermore, it has been demonstrated that an imbalance of glutamate neurotransmission plays a critical role in the pathophysiology of various neurological and psychiatric diseases.
The excitatory neurotransmission of glutamate is mediated through at least two different classes of receptors, the ionotropic glutamate receptors (NMDA, AMPA and kainate) and the metabotropic glutamate receptors (mGluR). The ionotropic receptors are ligand gated ion channels and are thought to be responsible for the regulating rapid neuronal transmission between two neurons. The metabotropic glutamate receptors are G-protein coupled receptors (GPCRs) which appear to mediate not only synaptic transmission, but also to regulate the extent of neurotransmitter release as well as post synaptic receptor activation.
Dysregulation in glutamatergic neurotransmission, for example through altered glutamate release or post-synaptic receptor activation, has been demonstrated in a variety of neurological as well as psychiatric disorders. Hypofunction of the NMDA receptor has not only been demonstrated in Alzheimer's patients, but is increasingly accepted as the putative cause of schizophrenia (Farber et al., Prog. Brain Res., 1998, 116: 421-437, Coyle et al., Cell. and Mol. Neurobiol. 2006, 26: 365-384). This is supported by clinical studies showing that antagonists of the NMDA receptor induce symptoms indistinguishable to those suffered by schizophrenia patients (Javitt et al., Am J. Psychiatry, 1991, 148: 1301-1308). Therefore, approaches that could potentiate or normalize NMDA receptor signaling have the potential to treat neurological and psychiatric disorders.
mGluR5 belongs to a superfamily of currently eight identified Type III GPCRs, which are unique in that the glutamate ligand binds to a large extracellular amino-terminal protein domain. This superfamily is further divided into three groups (Group I, II and III) based on amino acid homology as well as the intracellular signalling cascades they regulate (Schoepp et al., Neuropharma, 1999, 38:1431-1476). mGluR5 belongs to group I and is coupled to the phospholipase C signalling cascade which regulates intracellular calcium mobilization.
In the CNS, mGluR5 has been demonstrated to be expressed mainly in the cortex, hippocampus, nucleus accumbens and the caudate-putamen. These brain regions are known to be involved in memory formation and cogntive function as well as emotional response. mGluR5 has been shown to be localized post-synaptically, adjacent to the post-synaptic density (Luj an et al., Eur. J. Neurosci. 1996, 8: 1488-1500). A functional interaction between mGluR5 and the NMDA receptor has also been demonstrated, where activation of mGluR5 potentiates the activation state of the NMDA receptor (Mannaioni et al, NeuroSci., 2001, 21:5925-5924, Rosenbrock et al., Eur. J. Pharma., 2010, 639:40-46). Furthermore, activation of mGluR5 has been demonstrated in pre-clinical in vivo models to rescue cognitive impairment as well as psychotic disturbance induced by NMDA receptor antagonists (Chan et al., Psychopharma. 2008, 198:141-148). Therefore, activation of mGluR5, and thereby potentiation or normalization of the NMDA receptor signaling, is a potential mechanism for the treatment of psychiatric and neurological disorders.
Most agonists of mGluR5 bind the orthosteric glutamate binding site. Since the glutamate binding site between the mGluR family members is highly conserved, it has been challenging to develop selective mGluR5 agonists which have acceptable CNS penetration and demonstrate in vivo activity. An alternative approach to achieve selectivity between the mGluR family members is to develop compounds which bind to an allosteric site, which is not as highly conserved between the family members. These allosteric binding compounds would not interfere with the natural glutamate binding and signaling, but modulate the receptor activation state.
Positive allosteric modulators of mGluR5 have recently been identified (O'Brien et al., Mol. Pharma. 2003, 64: 731-740, Lindsley et al., J. Med. Chem. 2004, 47: 5825-5828). These compounds potentiate mGluR5 activity in the presence of bound glutamate. In the absence of bound glutamate, the mGluR5 positive modulators do not demonstrate intrinsic activity. Therefore, these compounds potentiate the natural signaling of mGluR5 as opposed to agonists which activate the receptor in a permanent, unnatural manner. mGluR5 positive allosteric modulators therefore represent an approach to potentiate mGluR5 signaling which in turn potentiates and normalizes the NMDA receptor hypofunction detected in neurological and psychiatric disorders.